A conventional ultrasonic actuator is shown in FIGS. 13 and 14. FIG. 13 is a perspective view of a piezoelectric element of the conventional ultrasonic actuator and FIG. 14 is a sectional view of the same.
A piezoelectric element 100 is supported by five supporting parts 101A, 101B, 101C, 101D and 101E. The piezoelectric element 100 includes four quadrant electrodes 102a, 102b, 102c and 102d formed on a surface of the piezoelectric element 100 and an overall electrode (not shown) formed to cover the entire area of an opposite surface of the piezoelectric element 100.
A wire 104a is connected to the electrode 102a by a solder 105a and to the electrode 102d by a solder 105d. A wire 104b is connected to the electrode 102b by a solder 105b and to the electrode 102c by a solder 105c. Further, a wire 104g is connected to the overall electrode. A voltage is applied to the piezoelectric element 100 through the wires 104a, 104b and 104g. 
A driver element 102 is provided on the top surface of the piezoelectric element 100 and a head of the driver element 102 is in contact with a movable object 103. The head of the driver element 102 is pressed onto the movable object 103 by the supporting part 101C. As a result, friction between the head of the driver element 102 and the movable object 103 is increased such that the vibration of the piezoelectric element 100 is surely transmitted to the movable object 103 via the driver element 102.
Hereinafter, how to operate the ultrasonic actuator is briefly explained.
FIGS. 15, 16 and 17A to 17D are conceptual diagrams illustrating the vibration modes of the piezoelectric element.
With the wire 104g connected to ground, a sinusoidal reference voltage of a certain frequency is applied to the wire 104a and a voltage having a phase shifted by 90° or −90° relative to the reference voltage is applied to the wire 104b. Accordingly, the piezoelectric element 100 is induced to vibrate in a second-order mode of bending vibration shown in FIG. 15 and a first-order mode of stretching vibration (so-called longitudinal vibration; hereinafter may be referred to as longitudinal vibration) shown in FIG. 16.
Resonance frequencies of the bending and stretching vibrations are determined by the material and shape of the piezoelectric element 100. When the two resonance frequencies are set almost equal and a voltage having a frequency near the set frequency is applied, the piezoelectric element 100 is induced to vibrate in a second-order mode of bending vibration and a first-order mode of stretching vibration in a harmonious manner. Thus, the shape of the piezoelectric element 100 varies sequentially in the order shown in FIGS. 17A to 17D.
As a result, the driver element 102 provided on the piezoelectric element 100 makes an elliptical motion as viewed in the direction perpendicular to the page surface. That is, the bending and stretching vibrations of the piezoelectric element 100 are combined to cause the elliptical motion of the driver element 102. Due to the elliptical motion, the movable object 103 supported by the driver element 102 moves in the direction of an arrow A or B. Thus, the function of the ultrasonic actuator is achieved.
Further, as shown in FIG. 18, another piezoelectric actuator has been proposed which includes a rectangular piezoelectric element 110 and a plurality of substantially hemispherical driver elements 112.
As a known prior art document related to the invention of the present application, for example, we note Patent Literature 1: Japanese Unexamined Patent Publication No. 2004-304963.